Filter devices for aircraft hydraulic circuits are known that comprise at least one cartridge filter that is separately fitted to a hydraulic block. The filter comprises a vessel that receives a filter cartridge. The vessel is screwed to the hydraulic block and is tightened with torque that is sufficient to ensure that the thread holds together in service, and in particular that avoids any fretting.
Given the dimensions being proposed for aircraft, such vessels are reaching considerable dimensions, and that the tightening torque increases in proportion, thereby leading to various problems. Firstly, delivering such torque requires torque wrenches to be used that are large in size and heavy, difficult to handle, and above all in zones that are sometimes difficult of access in bays of the aircraft. Such wrenches also require the operator to deliver considerable force. Furthermore, such a tightening torque leads to high levels of stress on the internal thread in the hydraulic block. Given the fatigue cycling that is caused by the vessel being screwed on and off, these high levels of stress constitute a risk of the internal thread in the hydraulic block deteriorating. These risks are made worse by the fact that the hydraulic block is generally made of a material such as aluminum or titanium in which internal threads are known to wear rapidly, even when protective treatments are used.